1. Field of the Invention
The present invention relates to compositions that include fluoropolymers.
2. Description of the Related Art
Fluoropolymers are well known materials. As used herein the term xe2x80x9cfluoropolymersxe2x80x9d includes polymers which are made from at least one fluorine-containing monomer, but may incorporate monomer(s) which contain no fluorine or other halogen. The fluoropolymer, however, preferably contains at least 35 weight percent fluorine. Examples of fluoropolymers include polymers such as polytetrafluoroethylene (PTFE), perfluoroalkoxy (PFA), fluorinated ethylene propylene (FEP), polyvinylidene fluoride (PVDF), ethylene-tetrafluoroethylene (ETFE), ethylene-chlorotrifluoroethylene (ECTFE), MFA, THV, and mixtures thereof. These polymers may contain one or more monomers in addition to those named. MFA is made from tetrafluoroethylene and perfluoromethylvinylether, and may also include perfluoropropylvinylether monomer. MFA is available from Ausimont S.p.A. THV is made from tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride, and is available from Dyneon. Fluoropolymers possess a unique combination of properties such as high temperature resistance, low electrical conductivity, excellent chemical compatibility, non-stick, low surface tension, and the like. Accordingly, fluoropolymers are often added to other materials, such as other polymers, in order to improve certain physical properties of those materials.
Fluoropolymer dispersions may be used in applications such as coatings and impregnation, or may be coagulated and dried into powders, and eventually extruded into pellets. These powders or pellets may be made into articles such as tubes or insulation for electrical wire and cables by melt or paste extrusion. Alternatively, these powders or pellets may be used as additives in plastics, coatings, and the like.
One type of a fluoropolymer that is sometimes added to other polymers is high molecular weight PTFE polymer powder. High molecular weight PTFE polymer has a tendency to easily agglomerate and fibrillate under shear and thus is difficult to transport and difficult to feed into host materials. Furthermore, high molecular weight PTFE polymer can not be manufactured in small particle sizes. Generally speaking, the polymer produced in known manufacturing processes has a particle size of well above an average of 30 microns.
In order to obtain a homogeneous dispersion of PTFE in a host such as another polymer, an aqueous dispersion of PTFE is sometimes used. The PTFE has average particle sizes of 0.2 to 0.3 microns. If the presence of water is undesirable, then these aqueous dispersions are dried to form fine powder resins. The problem with these powders is that they tend to easily agglomerate and be deformed. The average particle size of the powder is at least 300 microns which causes poor distribution of the PTFE within the host, thereby lowering the efficiency of the PTFE and raising the cost of the final product because a greater amount of PTFE is needed in order to obtain its beneficial effects. Further, PTFE tends to fibrillate, which makes it useful as a drip suppressant in a burning host polymer. However, the PTFE particles obtained by drying an aqueous dispersion may fibrillate prematurely, decreasing the effectiveness of the PTFE as a drip suppressant.
Fluoropolymers such as PTFE are also added as coatings to the surface of certain types of films, such as industrial films made of ethylene copolymers, to reduce the friction between the film and processing equipment, thereby increasing the speed of operation of the equipment. However, such coatings may be from only a micron to a few microns thick, and the PTFE particles are much larger which leads to roughness of the film, poor distribution of PTFE in the film, and the likelihood that the PTFE will easily come off from the film.
What is needed, therefore, is a way to obtain fine powder PTFE particles which do not have the prior art problems of agglomeration, deformation and premature fibrillation and which distribute homogeneously in host materials. Further, there is needed a way to add fine powder PTFE particles to coatings such as films.
One could incorporate the fluoropolymer in the bulk of the host film, but this would increase the costs substantially and unnecessarily, since the desired effect is exclusively on the surface. Furthermore, the presence of the fluoropolymer tends to lower the sealing strength of the bonded films.
What is needed, therefore, are ways to add fluoropolymers such as PTFE to other polymers, to films and other substrates taking advantage of the special properties of fluoropolymers.
This invention relates to an aqueous dispersion of (a) from 0.5 to 99.5 weight percent of a fluoropolymer having an average particle size of from 0.01 to 30 microns; and (b) from 0.5 to 99.5 weight percent of an ethylene copolymer. These weight percents are based on the total amounts of (a) and (b) only. The ethylene copolymer comprises at least 50% by weight ethylene, 1-35% by weight of an acid-containing unsaturated mono-carboxylic acid, and 0-49% by weight of a moiety selected from at least one of alkyl acrylate, alkyl methacrylate, vinyl ether, carbon monoxide and sulfur dioxide, and further wherein the acid groups are neutralized from 0-100% by a metal ion or other cations.
The invention may be used in the form of this dispersion, or in the form of a coprecipitate of the dispersion. Also, the coprecipitate may be dried to form a dry powder and used in that form or then pelletized to be used as pellets.
The compositions of the invention may be added to a host such as a polymer to make an extruded or molded article or to make a film.